Last year when I was really upping the climbing on my long training rides on the weekends, I had a few rides when afterward I was just TRASHED. I live in a cold, damp climate. And, on these rides, I dressed as well as possible (in ways that would keep me warm on the bike), ate religiously with a balance of protein bars, sugar gels, electrolyte tabs, real food, etc. All stuff I know from experience works for me and keeps me going on rides.
But, I was just very much upping the difficulty of my endurance rides. I would get in a zone, and not bonk on the ride, and even feel like I could keep going when I got home.
BUT...once I stopped, and got off the bike, I sometimes got very cold, and just downed a huge glass of sugary chocolate milk, and got in a hot epsom bath asap. My leg muscles would just ACHE. And, sometimes it would take a half hour to warm up in a hot bath.
THEN...bathrobe, regular meal, etc, and I would start feeling like a human being again.
"The best rides are the ones where you bite off much more than you can chew, and live through it." ~ Doug Bradbury